Abstract

With the aim to enhance the electrochromic (EC) efficiency and electrochemical stability of electrochromic devices (ECD), mixed nanostructured TiO2/WO3 films were prepared by an electrochemical deposition method with the purpose of adding WO3 nanoparticles to porous nanocrystalline doctor-blade TiO2 (nc-TiO2) films. The results of the characterization of electrochromic properties in 1 M LiClO4 + propylene carbonate (LiClO4 + PC) of both the nc-TiO2/F-doped tin oxide (FTO) and WO3/TiO2/FTO configurations showed the reversible coloration and bleaching of the ECDs. The response time of the ECD coloration of WO3/TiO2/FTO was found to be as small as 2 sec, and its coloration efficiency (CE) as high as 35.7 cm2 × C−1. By inserting WO3 nanoparticles into the porous TiO2 structures, WO3/TiO2 heterojunctions were formed in the films, consequently enabling both the CE and electrochemical stability of the working electrodes to be considerably enhanced. Since a large-area WO3/TiO2 can be prepared by the doctor-blade technique followed by the electrochemical deposition process, mixed nanostructured Ti-W oxides electrodes constitute a good candidate for smart window applications, taking advantage of the excellent coloration and stability properties as well as the simple and economical fabrication process involved.

1. Introduction

Electrochromism is a topic that has attracted a great deal of interest from researchers because of its potential application in various areas (photonics, optics, electronics, architecture, etc.). Electrochromic (EC) properties can be found in almost all the transition-metal oxides, and their properties have been investigated extensively in the last decades [1]. These oxide films can be coloured anodically (Ir, Ni) or cathodically (W, Mo); however, WO3 is clearly the preferred material for applications. This is principally due to the fact that WO3-based electrochromic devices (ECDs) have normally a faster response time to a change in voltage and a larger coloration efficiency (CE) as compared to devices based on other electrochromic materials. Recently, Granqvist et al. [2] have made a comprehensive review of nanomaterials for benign indoor environments. In this paper, the authors show the characteristic data for a 5 × 5 cm2 flexible EC foil incorporating WO3, and NiO modified by the addition of a wide bandgap oxide such as MgO or Al2O3, PMMA-based electrolyte, and ITO films. Durability of the EC devices was demonstrated in performing several tens of thousands of coloration/bleaching cycles, and the device optical properties were found to be unchanged for many hours. To improve further the electrochromic properties, Ti-doped WO3 films were deposited by cosputtering metallic titanium and tungsten in an Ar/O2 atmosphere [3]. Another way to improve electrochromic properties of thin films is to use nanostructured crystalline films. For instance, nanocrystalline WO3 films were prepared by the organometallic chemical vapour deposition (OMCVD) method using tetra(allyl)tungsten. The size of grains found in these films was estimated by atomic force microscope (AFM) and scanning electron microscope (SEM) to be 20 ÷ 40 nm. The coloration of WO3 deposited on indium tin oxides (ITO) substrates (WO3/ITO) in 2 M HCl was less than 1 sec and the maximum coloration efficiency at 630 nm was 22 cm2 × C−1 [4]. However, the HCl electrolyte is not suitable for practical use. The Au-doped WO3 films were made by a dip-coating technique [5]. With fabrication of nanostructured WO3 films, Beydaghyan et al. [6] have shown that porous and thick WO3 films can produce a high CE. The open structure, fast response, and high normal state transmission made them good candidates for use in practical applications. The nanocrystalline TiO2 anatase thin films on ITO, prepared by sol-gel dipping method, exhibited a good reversible coloration and bleaching process [7], but the response time was slow and the electrochromic efficiency was not large. Recently [8], we have shown that by using the so-called “doctor-blade” method, nanoporous TiO2 anatase films on F-doped tin oxide (FTO) substrates (nc-TiO2/FTO) were prepared for the ECDs with much improved electrochromic response time and coloration efficiency (CE). However, with such porous films, for a long exposed performance time, the durability of the devices was limited, making the resulting ECD less satisfying for smart windows applications. In this work, with the aim to enhance the stability of the EC devices on one hand by making more tightly knitted films, and on the other hand, to improve the CE of the ECD performance with the help of nanostructured heterojunctions of TiO2/WO3, we combined a doctor-blade technique for preparing large-area films with an electrochemical process for depositing WO3 films into the nanoporous TiO2 structures.

2. Experimental

To prepare nanostructured TiO2 films for ECD, a doctor-blade technique was used following the process reported in [8]. A glass slide, overcoated with a 0.2μm thick FTO film having a sheet resistance of 15 Ω and a transmittance of 90%, was used as a substrate; the useful area that constitutes the sample studied was of 1 cm2. A colloidal solution of 15 wt% nanoparticles (15 nm in size) of titanium oxide (Nyacol Products) in water was used. For producing thinner films, we added more distilled water to get ca. 5 wt% TiO2 and a few drops of the liquid surfactant were added. Then the diluted solution was filled in the slot on the FTO electrode and spread along the tapes. The samples were left for drying during 15 min, then put to a furnace maintained at 450°C for 1 hour to recrystallize the nc-TiO2 films.

To deposit WO3 into nc-TiO2/FTO, the peroxy-tungsten acid solution (H2W2O11) was prepared. For this 4.6 g of high-purity metallic tungsten powder was dissolved in 25 ml H2O2 (30%). The excess H2O2 was decomposed by putting a large-area Pt sheet into the solution until gas evolution was stopped. After that, the solution was diluted to 50 mM by addition of water to form 500 mL of the solution and kept in cool air to avoid decomposition of peroxy tungstate to trioxide and polytungstate [9]. A standard three-electrode cell has been used: the working electrode was F-doped-SnO2 coated glass (FTO) with a sheet resistance of 15 Ω the reference electrode was saturated calomel electrode (SCE); a Pt grille was used as counter electrode. By using potentiostat “Auto lab. PGS-30,” a potentiostatic method was chosen for film deposition. After their deposition, the films were dried in nitrogen gas. Electrochromic properties of the films were determined by cyclic voltametry in electrolytic solutions of 1 M LiClO4 + Propylene carbonate (PC).

The film thickness was measured by an Alpha-step profiler and its surface morphology examined with the help of a field-emission scanning electron microscope (FE-SEM). X-ray diffraction analysis (XRD) was done on a Brucker “Advance-8D” X-ray diffractometer with X-ray Cu wavelength 𝜆 = 0.154 nm. Electrochemical processes were carried-out on the same potentiostat. All measurements were executed at room temperature.

By using a JASCO “V-570” photospectrometer, in situ transmittance spectra of nc-TiO2 in LiClO4 + PC versus bias potential were recorded on the films of the working electrode mounted in a modified electrochemical cell which was placed under the pathway of the laser beam, and the three cell electrodes were connected to a potentiostat.

3. Results and Discussion

3.1. Electrochemical Deposition

Under an application of the potential of −1.5 V/SCE, the fact that the current density was increased with the increase in time would prove that W6+ was inserted into the working electrode with a compensation of negative charges (e−). Due to the bonding of W6+ with O2− ions, WO3 was formed. This further was confirmed by XRD analysis (see the next section). From all the deposition experiments, it was seen that the current density increased fast and reached a saturate value in 40 sec (Figure 1).

Figure 1: The time dependence of the current density during electrochemical deposition of WO3 under a negatively polarized potential of −1.5 V/SCE.

With this interval, the WO3 volume almost filled up the pores in the porous TiO2 film, and then stopped increasing at the saturate current density. However, the total time of 130 sec for the deposition was chosen. The thickness of as-deposited WO3/TiO2 films, checked on the Alpha-Step Profiler, was of ~570 nm. These films were slightly thicker than the initial doctor-blade nc-TiO2 films (namely 550 nm).

3.2. Morphology and Crystalline Structure

In the doctor-blade deposition, the thickness of the films was found to be depending on preparation conditions such as the concentration of solutions and the spread speed. The samples used for further investigation were taken from films chosen with a concentration of 5 wt% TiO2 in water and a spread speed of 8 mm/s. A bright-field micrograph of the film surface is shown in Figure 2(a). The film is quite uniform, but some crystallized nanoparticles are a little larger than the initial TiO2 particles dispersed in water (namely 20 nm in size). The thickness of the films was of ca. 550 nm. In comparison with the nanostructured films prepared by sol-gel method [7], these films are thicker and much more porous. Thus, between the nc-TiO2 particles, there are numerous nanoscale pores which would favour the insertion of ions like Li+, Na+, and/or W6+ into the films, when a polarized potential is applied on the working electrode (nc-TiO2/FTO). With the insertion of the Li+, one can obtain the ECD coloration, whereas the insertion of W6+ during the electrochemical deposition would result in the insertion of WO3 nanoparticles into the pores of the nanostructured TiO2 film. Figure 2(b) shows the FE-SEM of the surface of the TiO2 electrode after the electrochemical process in the H2W2O11 solution. During the deposition, we observed that the TiO2/FTO electrode was coloured and became deep-blue with the increase in the deposition time. This coloration can be attributed to the insertion of W6+ (the ions from the H2W2O11 electrolyte) into the TiO2 films according to the following equation:
TiO2(transparent)+xW3++xe−⟶WxTiO2(blue),(1)
where W6+ plays the role of Li+ in the well-known equation [10]:
TiO2(transparent)+xLi++e−⟶LixTiO2(blue).(2)

Figure 2: FE-SEM micrographs of as-prepared nc-TiO2 films (a) and after the electrochemical deposition process in a H2W2O11 electrolyte (b).

From our previous work [11], it was shown that an electrochromic WO3 film was deposited onto the FTO electrode by the electrochemical technique. Hereby, simultaneously with the EC coloration, one can deposit WO3 particles into TiO2/FTO to form a film with heterojunctions of WO3/TiO2 created from a mixture of WO3 and TiO2 nanoparticles. This would lead to the different features observed in the FE-SEM picture of the WO3/TiO2 film (Figure 2(b)) as compared to the FE-SEM picture for a nc-TiO2 film (Figure 2(a)).

The crystalline structure of the films was confirmed by using an accessory for films with a small angle of the X-ray incident beam. For such a thick film of nc-TiO2, all XRD patterns of the FTO substrate do not appear (Figure 3(a)). XRD patterns show all the diffraction peaks corresponding to titanium oxide. Indeed, for the doctor-blade film, in the XRD patterns, there are the most intense peak of the (101) direction corresponding to 𝑑=0.352 nm and three smaller peaks corresponding to 0.238, 0.190, and 0.149 nm. These diffraction peaks are quite consistent with the peaks for a single crystal of TiO2 anatase.

Figure 3: XRD of a nc-TiO2 doctor-blade film (a) and of a WO3/TiO2 film coated by electrochemical deposition.

The fact that the peak width is rather small shows that the TiO2 anatase film was crystallized into large grains. To obtain the grain size 𝜏, we used the Scherrer formula:
𝜏=0.9𝜆,𝛽⋯cos𝜃(3)
where 𝜆 is wavelength of the X-ray used (𝜆 = 0.154 nm), 𝛽 the peak width of half height in radians and 𝜃 the Bragg angle of the considered diffraction peak [12]. From the XRD patterns, the half-height peak width of the (101) direction was found to be 𝛽 = 0.005, consequently, the size of (101) grain was determined as 𝜏≈20 nm. Similarly, the size for the other grains was found to be from 25 to 30 nm. This is in good agreement with data obtained by FE-SEM for the average size of particles where the crystalline grains were not identified by the Miller indexes (see Figure 2(a)).

The XRD patterns of the WO3/TiO2 (Figure 3(b)) possess both the peaks of TiO2 and WO3, where one peak of (004) direction of TiO2 was not revealed and relative intensity of the peaks changed a little bit. This is due to the presence of WO3 particles, whose average size, estimated by formula (1), is the same as the size of pores. This result, together with the observation of the FE-SEM pictures (Figure 1), demonstrates that WO3 particles were indeed filling-up the nanopores of the porous nc-TiO2 film.

3.3. Electrochemical Property

Figure 4 presents a chronoamperometric plot obtained by setting up five lapses of 5 sec for the coloration and bleaching, corresponding to −1.50 V/SCE and to +1.50 V/SCE, respectively. To calculate the inserted charge (𝑄) for the coloration state, we use the formula for integrating between the starting and ending times of each lapse of time as follows:
𝑄=𝑡2𝑡1𝐽(𝑡)𝑑𝑡.(4)

The insertion charge was found to be 𝑄in = 21 mC × cm−2, whereas for the extraction process the charge was 𝑄ex = 19 mC × cm−2, that is slightly different from the insertion charge. The fact that the insertion and extraction charges are similar proves the reversibility of the electrochromic coloration/bleaching process—a desired characteristic for the electrochromic performance of the WO3/TiO2-based electrochromic devices.

Figure 5 shows the cyclic voltammetry (CV) spectra of a WO3/TiO2/FTO electrode cycled in 1 M LiClO4 + PC. In the positive scanning direction (PSD) there are two peaks of the current density at potentials of −1.0 V/SCE and at −0.32 V/SCE. In the negative scanning direction (NSD) there are also two similar peaks. This proves the reversibility of the insertion/extraction of Li+ into/out of the WO3/TiO2/FTO electrode. Moreover, the CV spectra of the WO3/TiO2/FTO electrode possess a different characteristic from the one of each compound for either WO3/FTO or TiO2/FTO. These CV spectra can be seen as just the CV of a mixture of WO3 and TiO2 electrodes cycled simultaneously in LiClO4 + PC electrolyte.

Therefore the electrochromic performance of the WO3/TiO2/FTO electrode in LiClO3 + PC can be described by two cathodic reactions; one is [13, 14]:
WO3+xLi++e−⟷LixWO3(5)
and the other one is reaction (2).

3.4. Electrochromic Performance

For a sample with a 570 nm thick WO3/TiO2 film, the in situ transmission spectra, obtained during coloration at a polarized potential of −1.2 V/SCE are given in Figure 6.

Figure 6 shows the transmittance of a 570 nm thick WO3/TiO2 film in 1 M LiClO4 + PC at different polarizing bias potentials (from −0.5 V/SCE to + 0.3 V/SCE). From this figure, one can see a large difference in transmittance spectra in the visible range of the film between the colored (curve 1) and bleached (curve 9) states. The transmittance at 𝜆 = 550 nm (the sensitivity peak of human eye) from ~85% with the bleached state decreased to 40% with the colored state, corresponding respectively to bias potential from +0.3 V/SCE to −0.5 V/SCE. This result indicates that the electrochemically deposited WO3/TiO2 films exhibited an excellent EC performance that can be comparable to the one of the cosputtered Ti-doped WO3 films, as reported in [3]. The transmittance decreases with the increasing of the bias potential in NSD interval.

The bleaching and coloration processes were measured under the application of negatively and positively polarized voltage to the working electrode, respectively. These processes were clearly associated with the Li+ insertion (extraction) from the LiClO4 + PC electrolyte into (out of) the WO3/TiO2/FTO electrode. In comparison with results reported previously [8], the optical modulation at the visible range for WO3/TiO2/FTO at longer wavelengths is improved. Herein, we attained a transmittance at 𝜆 = 550 nm (T550) equal to 84% upon bleaching and to 42% after a coloration period of 30 s. The largest optical modulation in the visible range was observed for red light (T700) as seen in Figure 7: the gap between the transmittances of bleaching and coloration states was of 70%. For blue light (T400), the optical modulation at wavelength 400 nm was negligible, that is about 5%. This would result from the strong absorption by both WO3 and TiO2 at shorter wavelengths.

Figure 7: Time-dependence transmittance of the WO3/TiO2/FTO during electrochromic performance for three different wavelengths: 400, 550, and 700 nm.

From the above-mentioned results for the ECDs with heterojunctions of WO3/nc-TiO2, it is seen that the efficient coloration can be achieved due to a double-coloration process, such as shown in (2) and (5). To evaluate the electrochromic coloration efficiency (𝜂) we used a well-known expression relating the efficiency with the optical density (ΔOD), consequently the transmittances of coloration (𝑇𝑐) and bleaching states (𝑇𝑏), and the insertion charge (𝑄), as follows [15]:
𝜂=ΔOD𝑄=1𝑄𝑇ln𝑏𝑇𝑐.(6)

At a wavelength of 550 nm, 𝑄in = 21 mC × cm−2, 𝑇𝑏=85% and 𝑇𝑐 = 40%, the coloration efficiency was determined to be 35.7 cm2× C−1. The larger the wavelength is, the higher the coloration efficiency is. In the visible range of wavelengths, all the found values of 𝜂 are comparable to those for WO3 films [16] and much higher than those for TiO2 films [7] prepared by sol-gel techniques and titanium-lanthanide oxides deposited by magnetron sputtering and coloured in a LiClO4 + PC solution [17].

To investigate the ageing process of two types of electrochromic devices, the first one is made from nc-TiO2 and the second one from WO3/nc-TiO2 electrode, we measured the optical density versus bias potential after cycling these electrodes in 1 M LiClO4 + PC for 103 cycles. At a fixed wavelength, the larger is the difference between 𝑇𝑐(𝜆) of the colored state and 𝑇𝑏(𝜆) of the bleached state, the higher is the value of ΔOD. The potential dependence of ΔOD at 𝜆 = 550 nm for two above-mentioned electrodes is shown in Figure 8. From this figure, we can see that for both the mentioned electrodes, the best optical density has been found for the bias potential of −0.5 V/SCE. At this potential, the largest number of Li+ cations would be inserted into the electrodes making the TiO2 and WO3 structures change to LixTiO2 to LixWO3, respectively. The number of colour centers (namely LixTiO2 and LixWO3) is dependent on the number of Li+ cations and electrons injected into the electrodes. In general, this number increases proportionally to the bias potential applied on the working electrode. The reason why the ΔOD of the nc-TiO2 based ECD decreased faster than that of WO3/TiO2 is revealed by the monitoring of the film thickness of the electrodes during the electrochromic performance. After 103 cycles of the ECD performance, the thickness of both electrodes was checked with the Alpha step profiler, and it was found that a part of the nc-TiO2 film was removed while the WO3/nc-TiO2 was maintained almost unchanged. This demonstrates that WO3 attached to TiO2 particles enables the structure of WO3/nc-TiO2 film to be much more strengthened. Thus the devices with WO3/TiO2 nano-heterojunctions would possess a larger ECD lifetime.

4. Conclusion

Mixed nanostructured Ti-W oxides films were prepared by electrochemical deposition of WO3 onto doctor-blade nc-TiO2/FTO films. Electrochromic performance studies of both the nc-TiO2/FTO and WO3/TiO2/FTO configurations were carried out in 1 M LiClO4 + propylene carbonate. The reversible coloration and bleaching process of WO3/TiO2-based electrochromic devices was demonstrated. The response time of the ECD coloration of WO3/TiO2/FTO was found to be as small as 2 s and the coloration efficiency could be as high as 35.7 cm2 × C−1. In situ transmittance spectra of the WO3/TiO2/FTO working electrode showed the insertion/extraction of Li+ ions into/out of the electrode. By incorporating WO3 inserted between TiO2 nanoparticles, WO3/TiO2 heterojunctions were formed in the films. The resulting nanostructured film enables both the coloration efficiency and electrochemical stability of the working electrodes to be considerably improved.

Since a large-area WO3/TiO2 electrode can be prepared by the doctor-blade method, followed by an electrochemical deposition, this type of electrode constitutes a good candidate for smart window applications, taking advantage of its excellent properties in terms of EC efficiency and electrochemical stability, as well as the simplicity of the fabrication process.

Acknowledgment

This work was supported by the MOST of Vietnam through the Project on Fundamental Scientific Research for Applications; code: 1/2010/HD-DTNCCBUD (2010–2013).